The present disclosure relates generally to power controllers and control methods for switched mode power supplies, especially to power controllers suitable for operating a switched mode power supply in quasi-resonant mode.
Power converters or adapters are devices that convert electric power provided from batteries or power grid lines into power with a regulated voltage or current, such that electronic apparatuses are powered properly. For advanced apparatuses that are required to be environment-friendly, conversion efficiency of a power converter, defined as the ratio of the power that the power converter outputs to a load over the power that the power converter consumes, is always a big concern. The less the power consumed by a power converter itself, the higher the conversion efficiency. It is a trend for power supply manufactures to pursue higher and higher conversion efficiency.
Power converters operating in quasi-resonant (QR) mode are proved, in both theory and practice, to work efficiently. FIG. 1 shows a switched mode power supply 10 capable of operating in QR mode.
Bridge rectifier 20 performs full-wave rectification, converting the alternative-current (AC) power source from an AC mains outlet into a direct-current (DC) input power source VIN. The voltage of input power source VIN could have an M-shaped waveform or be substantially a constant. Power controller 26 could be an integrated circuit with pins connected to peripheral devices. Via a drive pin GATE, power controller 26 periodically turns ON and OFF a power switch 34. When power switch 34 is ON, a primary winding PRM of the transformer energizes; and when it is OFF, the transformer de-energizes via a secondary winding SEC and an auxiliary winding AUX to build up an output power source VOUT for load 24 and an operation power source VCC for power controller 26, respectively.
Resisters 28 and 30 form a voltage divider to detect voltage drop VAUX across the auxiliary winding AUX and to provide a feedback voltage signal VFB at a feedback pin FB of power controller 26.
FIG. 2 demonstrates waveforms of some signals in FIG. 1. Driving signal VGATE at drive pin GATE drops at time t0 to turn OFF the power switch 34, starting OFF time TOFF. Signal Vp at the joint P between the primary winding PRM and the power switch 34 raises sharply. Voltage drop VAUX, which is a reflective voltage in proportion to the voltage across the primary winding PRM, becomes positive suddenly at time t0. So does the feedback voltage signal VFB, which is a divided result of the voltage drop VAUX. The transformer starts de-energizing at time t0.
After the completion of de-energizing at time t1, voltage drop VAUX oscillates, substantially because of the resonant circuit substantially consisting of the primary winding PRM and any parasitic capacitors at the joint P. The waveform of voltage drop VAUX shown in FIG. 2 has three voltage valleys VL1, VL2 and VL3 where voltage drop VAUX is below 0V, and OFF time TOFF ends before valley VL3 completes. A power controller operating in QR mode operation turns on a power switch at a moment when a voltage valley occurs, and this skill is also referred to as valley switching. If the power switch 34 is turned on at the moment when voltage drop VAUX is at the bottom of a voltage valley, signal Vp is discharged from a local minimum, enjoying less switching loss. It is not always the case, however. A well-known conventional control method for a quasi-resonant switched mode power supply is to turn on a power switch after a constant delay time Td when a voltage valley starts. Exemplified in FIG. 2, the power switch 34 is turned on after voltage valley VL3 has started for a delay time Td. This constant delay time Td is generally a design choice, a constant fixed in an integrated circuit. Once the delay time Td is inappropriately chosen, the switching loss of the power switch 34 is not optimized.